materials Article Effect of Absorptivity of Superabsorbent Polymers on Design of Cement Mortars Jan Foˇrt 1,* , Przemysław Migas 1,2 and Robert Cernˇ ý 1 1 Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 7, 16629 Prague, Czech Republic; [email protected] (P.M.); [email protected] (R.C.)ˇ 2 Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31155 Cracow, Poland * Correspondence: [email protected]; Tel.: +420-77-501-3150 Received: 26 October 2020; Accepted: 30 November 2020; Published: 2 December 2020 Abstract: The functional properties of composites modified by superabsorbent polymers (SAPs) strongly depend on the swelling capacity of applied SAPs. In this sense, three types of commercially available SAPs namely Cablock CT, Hydropam, and Creasorb SIS with different chemical composition and particle size distribution were studied in this manuscript to reveal the differences in absorptivity as can be viewed as pretests for their utilization in concrete composites. In addition, absorptivity in distilled water, tap water, and 0.1 M NaCl solution are examined for determining the SAPs response for the change of the solution pH. To overcome problems with the teabag method inaccuracy, the new method is introduced. Besides to quantitative evaluation of the SAPs absorptivity, the correlation for the absorption and desorption period as the function of SAPs residence time within the examined solution is proposed. To access the effect of selected SAPs on functional properties, optimization based on the flow table test is employed and mechanical parameters are determined after 7, 14, 28, and 90 days of curing. Obtained results refer to substantial differences between particular SAPs and contribute to the understanding of the effect of SAP on the functional properties of cement-based materials. Keywords: superabsorbent polymer (SAP); absorptivity; teabag method; filtration method; mechanical performance; flow table test 1. Introduction Superabsorbent polymers (SAPs) are formed by hydrophilic networks that have to allow absorb, retain, and release (under changes in their environment), a significant amount of water or aqueous solution [1]. SAPs due to their unique properties are used in a plethora of applications, like hygiene and medical purposes [2–5], and as nutrient carriers, and water reservoirs applied in agriculture [6–8]. Another useful application can be found in concrete materials, where a number of possibilities are available due to their ability to control (absorption/release) free water in the cement-based mixture [9–11]. In this sense, SAPs have attracted increasing attention during the last decade due to the capability to mitigate drying shrinkage and promote self-healing in high-performance constructions [12]. Here, SAPs serve as small water reservoirs evenly distributed across the concrete volume, which can slowly release water during the cement hydration progress, and maintain optimal internal humidity during the curing stage or alternatively supply water to unreacted cement particles to boost self-healing [13]. Further water penetration from the environment may also contribute to a higher level of cement hydration [14,15]. Application of SAPs in concrete provides also improvement in the freeze/thaw resistance due to the formation of fine voids that play an important role in preventing internal pressure during water freezing [16]. Materials 2020, 13, 5503; doi:10.3390/ma13235503 www.mdpi.com/journal/materials Materials 2020, 13, 5503 2 of 16 Such modified materials show highly improved properties, by the meaning of mechanical strength, ductility, and durability [17]. Unfortunately, inaccurately selected dosages of SAPs taking into account the chemical composition and particle size may result in the worsening of functional properties of SAP modified cementitious materials [18]. On this account several research papers aimed at investigating the correlation between the chemical structure of SAP and the applicability in concrete mixtures as internal curing agents. Namely, the work of Tan et al. [19] discussed the dosing methods applied to support the beneficial effect of SAPs. As revealed, the addition of preswelled SAP led to a noticeable decrease in the compressive strength compared to the application of the powdered SAP. The variety of dosing methods were studied by Woyciechowski and Kalinowski [20], who contrary to the previously described study found the application of preswelled SAP as more efficient taking into account the preservation of mechanical performance. Since the swelling rate and capacity have been found as a crucial parameter for precise concrete design, the work of De Meyst et al. [14] highlighted the importance of the cross-linking degree of the SAPs. As noticed, the amount of additional water needed to obtain the same workability level of the mixture must be determined for each SAP material to avoid potential problems driven by a high swelling rate, hydration delay, and consequent deterioration of functional properties. The synergic effect of the application of SAPs and fly ash confirmed that Yang et al. [21] mentioned the synergistic effect of multiple ions. A more detailed elucidation of SAPs incorporation in cementitious materials can be found in the work of Mechtcherine et al. [22] who investigated the effect of molecular structure and applied solution on the autogenous shrinkage and mechanical strength of concrete mixtures. Considering the obtained findings, one can distinguish a correlation between SAP swelling and the ionic composition of applied liquids. Authors refer to a substantial influence of anion density on absorption and desorption behavior that closely correlates with the rheology of the fresh mixture [23]. Additionally, the kinetics of pore solution uptake and release during the concrete curing and maturing period in order to mitigate the autogenous shrinkage. On the other hand, the crosslinking density, as one of the parameters used for SAP description was not concluded as important for SAPs behavior. The research paper focused on the elucidation of the principles of SAPs application in cementitious materials with emphasis on the final strength in the hardened state based on the approximation of the Powers model [24,25]. The provided conclusions refer to the possible mitigation of high-performance concrete self-desiccation by the help of SAPs as agents for the formation of the desired pore distribution that promotes optimal curing conditions. On top of that, the frost resistance of such modified mixtures can be improved due to gas-filled voids produced during mixture preparation [26]. Mignon et al. [27] compared the self-healing capacity and compressive strength of obtained mortars. The research provides important information related to the application of commercial SAPs, and other types like modified methacrylate alginate combined with acrylic acid, and acrylamide, and methacrylate chitosan combined with the monomer dimethyl aminopropyl methacrylamide. Different SAP samples behave individually in cement-based materials depending on their chemical structure like cross-linking and functional group density and depending on their particle size distribution. Hence the proper use of SAP materials in cement mixtures requires knowledge of their basic properties. SAP absorptivity is one of the most important factors that characterized this type of material in terms of the efficient design of concrete composites. It translates into the rheological properties of the fresh concrete paste, which in turn is important in terms of its homogenization and pumping [28]. Understanding of the SAP absorption characteristic allows for proper dosage of additional water, which in addition to the aforementioned rheological properties, also translates into the mechanical properties of resulting concrete. This work presents the absorptivity of commercially available SAPs depending on the time, pH, and type of used solution. The presented results are the starting point for further research of SAPs application in cement-based materials. Materials 2020, 13, x 3 of 16 Materials 2020, 13, 5503 3 of 16 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. MaterialsThree SAPs with different particle size and chemical composition, namely Cabloc CT, Creasorb SIS, andThree Hydropam SAPs with (all di obtainedfferent particle from Evonik size and Industries chemical Ltd., composition, Essen, Germany), namely Cabloc were examined. CT, Creasorb All testedSIS, and polymers Hydropam are considered (all obtained as non from-degradable Evonik Industries and non Ltd.,-toxic Essen,materials Germany), [29]. Cabloc were isexamined. based on crossAll tested-linked polymers sodium arepolyacrylate, considered Creasorb as non-degradable on cross-linked and non-toxic acrylamide/acrylic materials [acid29]. Cablocpotassium is based salt, andon cross-linked Hydropam on sodium sodium polyacrylate, salt and acrylamide/acrylic Creasorb on cross-linked acid copolymer. acrylamide Used/acrylic SAPs differ acid potassiummainly in termssalt, and of Hydropamsolubility, while on sodium Cabloc salt and and Creasorb acrylamide do/acrylic not dissolve acid copolymer. in water, Used Hydropam SAPs di ffiser slightly mainly solublein terms and of solubility,creates a gel. while The Cabloc next difference and Creasorb between do notSAPs dissolve is the particle in water, size Hydropam distribution, is slightly which wassoluble analyzed and creates by the a laser gel. Theparticle next